1. Field of the Invention
The present invention relates to the production of alkylbenzaldehydes which are useful as raw materials or intermediates for perfumes, medicines and agricultural chemicals, more particularly, to the production of alkylbenzaldehydes from alkylbenzenes having a primary alkyl group of two or more carbon atoms.
2. Description of the Prior Art
The production process of an alkylbenzaldehyde by the reaction of an alkylbenzene and carbon monoxide in the presence of a catalyst such as hydrogen chloride-aluminum chloride is well know as Gattermann-Koch reaction. In this process, the regeneration of catalyst is very difficult because the reaction product mixture is usually treated with water after the reaction to facilitate the separation of the product and the catalyst. Another drawback of this process is an increased cost of waste disposal because a large amount of waste is produced by hydrolysis.
A modified Gattermann-Koch reaction using hydrogen fluoride and boron trifluoride as the catalyst is disclosed (U.S. Pat. No. 2,485,237 and JP 39-29760 B). Since hydrogen fluoride and boron trifluoride have high vapor pressures, hydrolysis is not needed to separate the product and the catalyst, this making it possible to recycle the catalyst for reuse. Thus, this process is one of industrially advantageous production methods of aromatic aldehydes.
The process of JP 39-29760 B proceeds according to the following reaction schemes when toluene is used as the starting material.
1. Preparation of solution of toluene/hydrogen fluoride-boron trifluoride complexCH3C6H5 (liquid)+HF (liquid)+BF3 (vapor)→CH3C6H5.H+BF4− (liquid)  (1)2. Formation of tolualdehyde/hydrogen fluoride-boron trifluoride complex by reaction with carbon monoxideCH3C6H5.H+BF4− (liquid)+CO (vapor)→CH3C6H4CHO.H+BF4− (liquid)  (2)
The formylation step (2) requires a higher pressure than required in the step (1) for preparing the solution of complex because the step (2) includes the reaction with carbon monoxide gas. In the proposed process, the supply of a large amount of boron trifluoride gas into a high-pressure reaction system is avoided by preparing the solution of toluene/hydrogen fluoride-boron trifluoride complex in advance.
It has been well known that the hydrogen fluoride-boron trifluoride catalyst is a good catalyst for the formylation on the one hand, but, unfavorably has an extremely high catalytic action for the disproportionation reaction (hereinafter merely referred to “disproportionation”) of alkylbenzenes on the other hand (J. Am. Chem. Soc., 75, 2411 (1953).
The disproportionation is particularly remarkable in alkylbenzenes having an alkyl group of two or more carbon atoms. For example, ethylbenzene is disproportionated as follows.

The disproportionation rapidly proceeds even at low temperatures as low as −20° C. and is difficult to prevent. However, an alkylbenzaldehyde resulted from the formylation of an alkylbenzene which is susceptible to disproportionation is stable against disproportionation even in the presence of the hydrogen fluoride-boron trifluoride catalyst. Utilizing this character, a process for producing an alkylbenzaldehyde at high yields has been proposed (JP 62-34024 B), in which the formation of the complex of alkylbenzene and hydrogen fluoride-boron trifluoride and the formylation by carbon monoxide are continuously carried out in a single step to prevent side reactions while introducing an alkylbenzene, hydrogen fluoride-boron trifluoride and carbon monoxide at a partial pressure of 5 kg/cm2 (about 0.5 MPa) or higher into a reactor.
Unlike the process comprising the schemes (1) and (2), however, this process needs the supply of a large amount of boron trifluoride gas into a high-pressure reaction system, because the formation of the alkylbenzene/hydrogen fluoride-boron trifluoride and the formylation by carbon monoxide should be carried out in a single step. Since the reactor is pressurized by carbon monoxide, boron trifluoride should be forced into the reactor after compressed by a gas compressor. Since boron trifluoride is a corrosive gas, the maintenance and inspection of the compressor require much labor.